KERNEL-MODE

The kernel-mode programs run in the background, making sure
everything runs smoothly - things like printer drivers,
display drivers, drivers that interface with the monitor,
keyboard, mouse, etc. These programs all run in such a way
that you don't notice them.

When the computer boots up, Windows calls the KERNEL, the
main kernel-mode program that allows all the other programs
to run, even the user-mode programs.

USER-MODE

These are the programs that you run when you want specific
programs - e.g., MS Paint, MS Word, and Calculator. These
are heavily restricted, as to not crash the system. Windows
uses memory-protection services offered by the processor to
prevent malicious programs from interfering with the rest of
the system and corrupting it.

A process can run in two modes:
    1.User Mode.     
    2.Kernel Mode.
1.User Mode:
  =>A mode of the CPU when running a program.
  =>In this mode ,the user process has no access to the
memory locations used by the kernel.When a program is
running in User Mode, it cannot directly access the kernel
data structures or the kernel programs.

2.Kernal Mode:
  =>A mode of the CPU when running a program.
  =>In this mode, it is the kernel that is running on behalf
of the user process and directly access the kernel data
structures or the kernel programs.Once the system call
returns,the CPU switches back to user mode.

    
When you execute a C program,the CPU runs in user mode till
the system call is invoked.In this mode,the user process has
access to a limited section of the computer's memory and can
execute a restricted set of machine
instructions.however,when the process invokes a system
call,the CPU switches from user mode to a                  
more privileged mode ­the kernel.  In this mode ,it is the
kernel that runs on behalf of the user process,but it has
access to any memory location and can execute any machine
instruction.  After the system call has returned,the CPU
switches back to user mode.

KERNEL-MODE

Kernel mode, also referred to as system mode, is one of the
two distinct modes of operation of the CPU in Linux. When
the CPU is in kernel mode, it is assumed to be executing
trusted software, and thus it can execute any instructions
and reference any memory addresses (i.e., locations in
memory). The kernel (which is the core of the operating
system and has complete control over everything that occurs
in the system) is trusted software, but all other programs
are considered untrusted software.

USER-MODE

User mode is the normal mode of operating for programs, web
browsers etc. They don't interact directly with the kernel,
instead, they just give instructions on what needs to be
done, and the kernel takes care of the rest. Kernel mode, on
the other hand, is where programs communicate directly with
the kernel. A good example of this would be device drivers.
A device driver must tell the kernel exactly how to interact
with a piece of hardware, so it must be run in kernel mode.
Because of this close interaction with the kernel, the
kernel is also a lot more vulnerable to programs running in
this mode, so it becomes highly crucial that drivers are
properly debugged before being released to the public.